Ind. Eng. Chem. Res. 2009, 48, 3293–3302
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Investigation of Batch Alkene Epoxidations Catalyzed by Polymer-Supported Mo(VI) Complexes Krzysztof Ambroziak,† Rene Mbeleck,‡ Yue He,† Basudeb Saha,*,† and David C. Sherrington‡ Department of Chemical Engineering, Loughborough UniVersity, Loughborough, Leicestershire, LE11 3TU, U.K., and WestChem Graduate School, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Glasgow, G1 1XL, U.K.
Polymer-supported Mo(VI) catalysts (PBI.Mo and Ps.AMP.Mo) have been prepared and characterized. The activities of both of these catalysts in epoxidation of alkenes (e.g., cyclohexene; 1-octene; limonene; R-pinene) with dry tert-butyl hydroperoxide (TBHP) as oxidant have been studied under different reaction conditions in a batch reactor. The long-term stability and Mo leaching of each polymer catalyst was also assessed by recycling a sample in batch reaction using conditions that will form the basis of a continuous process. Both catalysts have been shown to be sufficiently stable to be considered as real candidates for use in such a process. In the case of the PBI.Mo catalyst, the order of reactivity of the alkenes was found to be the following: cyclohexene > limonene > R-pinene > 1-octene. “Wet” TBHP can also be used for cyclohexene epoxidation catalyzed by PBI.Mo without any significant decrease in the final TBHP conversion. In the case of limonene epoxidation, however, the presence of water reduced the TBHP conversion and also affected the cis-trans ratio of the main product 1,2-limonene epoxide. The reusability studies with PBI.Mo revealed that the largest fall in the final TBHP conversion was observed for R-pinene epoxidation among the alkenes studied. For 1-octene epoxidation, the loss of Mo from each support has been investigated by isolating any residue from the reaction supernatant solutions, following removal of the heterogeneous polymer catalyst, and then using the residues as potential catalysts in epoxidation reactions. 1. Introduction Research into the immobilization of catalytically active metal species on organic polymers has been carried out over the last several decades.1-4 High long-term stability of polymersupported catalyst is as important as high activities and selectivities and is necessary for applications in continuously operating processes. There has been a lot of attempts to immobilize molybdenum on various supports either inorganic (silica, zeolites, layered double hydroxides), organic-inorganic (sol-gel-derived hybrid), or organic (polymers, ion-exchange resins).5-14 There has also been a considerable amount of academic work published on polymer-supported metal complex alkene epoxidation catalysts with a number of Mo(VI) based systems looking particularly attractive for scale-up and commercial exploitation.15-21 Despite this there appears to have been no significant attempt to move the chemistry on from being a useful small-scale laboratory procedure to being a process for medium- to large-scale production technology. Indeed this is the situation with essentially all the attractive polymer-supported catalysts that have been developed in the past decade or so. One of the reasons for this is the concern that in continuous processes apparently long-lived heterogeneous catalysts may prove to be unstable with even low levels of metal leaching causing more problems than are solved. We are now involved in a chemistry/chemical engineering collaboration aimed at using active and selective polymersupported Mo(VI) alkene epoxidation catalyst employing t-butyl hydroperoxide (TBHP) as the oxidant for continuous epoxidation in a reactive distillation column (RDC) upon which we will * To whom correspondence should be addressed. E-mail: B.Saha@ lboro.ac.uk. Tel.: +44 (0)1509222505. Fax: +44(0)1509223923. † Loughborough University. ‡ University of Strathclyde.
report in due course. In the meantime, we have carried out extensive batch studies to identify two potential catalyst candidates which might be used in a continuous process and to give a indication of the likely optimal conditions that might be used with particular alkenes. The first candidate is polybenzimidazole (PBI) supported molybdenum (VI) (PBI.Mo), which one of us developed for use in small batch reactions some time ago.16 The other candidate is polystyrene 2-(aminomethyl)pyridine supported molybdenum (VI) complex (Ps.AMP.Mo) which has also recently been proved to be a very active catalyst in cyclohexene epoxidation.15 A detailed evaluation of the catalyst reusability and Mo leaching has been conducted in the present study. This employs a technique for checking the catalytic activity in any residues found in the supernatant solutions of reactions following removal of the heterogeneous catalyst. This is a particularly powerful approach in the case of highly potent (heterogeneous) catalysts when active catalyst residues are suspected to be present below the detection limit of available analytical techniques. In batch studies, the catalysts have also been examined under different reaction conditions and with a variety of alkenes using TBHP as an oxidant. As far as alkene representatives, we have chosen cyclohexene, a relatively reactive alkene, and 1-octene as a less reactive terminal alkene.26 We have also studied the epoxidation of terpenes (e.g., limonene and R-pinene) as the terpene epoxides are valuable materials for perfumes, synthetic flavorings, and pharmaceuticals.27,28 2. Experimental Details All the chemicals used for this study except for the PBI were purchased from Aldrich Chemical Co. Inc., and they were used without further purification. The PBI was a gift from the Celanese Corporation.
10.1021/ie801171s CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
3294 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 1. Mo Loading and Mo/Ligand Ratio and Physical Properties of Polymer-Supported Mo Catalysts properties a
Mo loading (mmol Mo/g resin) ligand loading (mmol/g resin)b ligand/Mo ratio particle size BET surface area
PBI.Mo catalyst
Ps.AMP.]Mo catalyst
0.95 2.3 2.4:1 10% below 228 µm; 90% below 331 µm 22.12 (( 0.3) m2/g
0.67 0.73 1.1:1 10% below 92 µm; 90% below 152 µm 36.43 (( 0.6) m2/g
a From AAS analysis of digested resins. b From N% elemental analysis of Mo loaded resins assuming ligand ) imidazole or aminomethyl pyridine as appropriate.
2.1. Preparation of Mo(VI) Polymer Supported Catalysts (PBI.Mo and Ps.AMP.Mo). Polybenzimidazole (PBI. Mo) and polystyrene 2-(aminomethyl)pyridine (Ps.AMP.Mo) supported Mo(VI) catalysts were prepared and activated according to the procedure described in our recently published paper.15 The method for determining the Mo content of supported catalyst and ligand/metal ratio was also described in the above reference. Particle size distribution measurement of the catalysts was carried out using a Malvern Mastersizer. BET surface area measurements were carried out by the nitrogen adsorption and desorption method using a Micromeritics ASAP 2010 (accelerated surface area and porosimetry). The metal loading data, corresponding ligand/metal ratio, and physical properties of polymer-supported Mo(VI) catalysts are presented in Table 1. 2.2. Preparation of tert-Butyl Hydroperoxide (TBHP). t-Butyl hydroperoxide (70%) from the Aldrich Chemical Co. was rendered anhydrous by Dean-Stark distillation from a toluene solution following the modified method18 that was previously reported by Sharpless and Verhoeven.29 The molarity of TBHP was determined by iodimetry.29 The concentration of the TBHP in toluene was found to be 3.65 mol/dm3. 2.3. Batch Epoxidation Studies. The experiments were conducted in a 250 mL jacketed batch sealed reactor equipped with a condenser, overhead stirrer, thermocouple, and sampling point. Alkene and TBHP in toluene were weighed out and introduced into the reactor (no cosolvent was added). The catalyst was added when the reaction mixture in the reactor reached the desired temperature. The time was noted when all the catalyst was added to the reactor. Samples were taken at regular time intervals. All the reactant and product compositions were analyzed by gas chromatography. An HP 5080 II gas chromatograph (GC) was used to analyze the composition of samples from the liquid phase of the reactive mixture. The GC was fitted with a 30 m long J&W DB-5 MS, 0.32 mm diameter, and 0.25 µm film capillary column with a flame ionization detector (FID). Both injector and detector temperatures were set at 473 K, and the helium carrier gas flow was maintained at 1 mL/min (split ratio 100). For the epoxidation of cyclohexene, 1-octene and R-pinene the column temperature was programmed between 313 and 473 K (313 K for 4.5 min, then ramped 25 K/min to 473 K). For the epoxidation of limonene, the column temperature was programmed between 323 and 473 K (323 K for 5 min, then ramped 4 K/min until 388 K, and finally ramped at 30 K/min to 473 K). 2.4. Recycling of Polymer-Supported Catalysts. In order to evaluate the long-term activity and stability of each polymersupported catalyst and also investigate the possible contribution, if any, to the overall catalysis from homogeneous Mo species leached from each polymer support, a sample of each catalyst was employed in 5-10 consecutive reactions. “Supernatant studies” were carried out by isolating the polymer catalyst from each catalytic run via filtration, and the corresponding super-
natant solution, potentially containing catalytically active Mo species, was evaporated to dryness. New epoxidation reaction components were added to any residues present, visible or not, and the reaction was carried out as before. 3. Results and Discussion 3.1. Effect of Reaction Temperature on TBHP Conversion Catalyzed by PBI.Mo. It was shown previously19 that PBI.Mo requires no activation step involving oxidation with TBHP for this complex to become catalytically active, whereas for polymer catalysts similar to Ps.AMP.Mo, unless these are preoxidized (activated), epoxidation can be preceded by a significant induction period. We have also tried to rationalize these observations in terms of the structure of the Mo complex on each of these supports.30 In the present work, therefore, it was decided to standardize the procedure for the two polymer catalysts and activate each for 4 h as indicated in section 2.1. It is to be noted that aerobic oxidation of alkenes involving PBI-supported metal complexes tends to yield allylic oxidation products via a free radical mechanism and that a mono-oxygen source such as TBHP is required to achieve alkene epoxidation via nonfree radical selective mechanisms.31 There is no significant allylic oxidation detected in the present work, and so, any participation by molecular oxygen as an oxidant seems minimal. The PBI support itself has been shown to be very oxidatively stable in many earlier publications, and indeed for this reason, the material is used as an aerospace polymer.30 It is also highly thermally stable and displays good resistance to Mo leaching. Simple thermal decomposition of TBHP under the conditions of the epoxidations carried out is negligible. For all alkenes except limonene, the selectivity toward alkene epoxides was found to be close to 100%. It is evident from Figure 1a that the epoxidation of cyclohexene catalyzed by the PBI.Mo complex is very fast. After 50 min of the reaction, nearly 100% conversion of TBHP was achieved at 353 K. A distinct exothermic effect was observed when the reaction was carried out at 353 K, with the temperature increasing to 363 K (in about 10 min). It was then controlled immediately to maintain the reaction mixture at temperature 353 K. Therefore, the significantly higher conversion and reaction rate at that temperature is probably due to the increase in temperature. Limonene, which is the main component of the orange essential oil, has a low price, and is thus a particularly important substrate for the production of much higher value products.28,32,33 One of the limonene oxidation products is limonene oxide, which is a key raw material in the synthesis of pharmaceuticals, fragrances, perfumes, and food additives.27,34 Most of the known solid catalysts, e.g. carbon-anchored Co(acac)2,35 Ti-MCM-41,36 and supported Keggin heteropoly compounds, show significantly low limonene conversions and epoxide selectivities (20-30%); carveol, carvone, and polymeric products are also formed in large quantities.37
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Figure 1. Effect of temperature on TBHP conversion and reaction rate of PBI.Mo catalyzed epoxidation of alkenes: (a) cyclohexene, (b) limonene (conversion of TBHP is related to 1,2-limonene epoxide), (c) R-pinene, (d) 1-octene (at a 5/1 molar ratio of alkene to TBHP, catalyst content 0.3 mol % Mo, 400 rpm agitation speed).
Wentzel et al.38 studied the use of a PBI.Mo catalyst in the aerobic epoxidation of limonene; however, no reaction was observed. In our studies using the same catalyst, with TBHP as an oxidant, the main product found in all cases is 1,2-limonene epoxide. GC analysis shows the 1,2-epoxide to be 50:50 mixture of cis and trans diastereoisomers. 8,9-Limonene epoxide is also found as a byproduct. Notably neither the diepoxide nor any other byproduct were detected during the present study. Regardless of reaction conditions, the selectivity of 1,2-limonene oxide reached 94% after 60 min and remained constant at higher conversion until the end of the experiment. The epoxidation of limonene at 353 K catalyzed by the PBI.Mo complex is not as fast as cyclohexene epoxidation. However, after 120 min of the reaction, nearly 90% of TBHP conversion to 1,2-limonene oxide is achieved (Figure 1b). The reaction rate for epoxidations carried out at 353 and 343 K is very similar while a significant drop of reaction rate at 333 K was recorded. However, at the end of the experiment (350 min) at 353, 343, and 333 K similar TBHP conversions to 1,2-limonene oxide were achieved for all three temperatures. An exothermic effect was observed especially when carrying out the reaction at 353 K. R-Pinene oxide is used as an intermediate in the production of synthetic substitutes for natural aromas. Rearrangements of R-pinene oxide can result in campholenic aldehyde and transcarveol, which are the constituents of the East Indian sandalwood and Valencia orange essence oils, respectively. Both of these oils are expensive ingredients in the flavor industry.39 Variation of temperature clearly markedly influences the TBHP conversion to R-pinene oxide which at the end of experiment at 353 K reaches 90% (Figure 1c). For the other temperatures studied the TBHP conversion drops down pro-
portionally giving 70% at 343 K and 50% at 333 K at the end of experiment. The reaction rate is not as fast as it is in case of cyclohexene and limonene epoxidations. The TBHP conversion achieved at the end of reaction time at 353 K for 1-octene epoxidation is the lowest obtained for the epoxidation of all the alkenes studied catalyzed by PBI.Mo. The fall in conversion with decrease in temperature is also very significant (Figure 1d). In order to achieve higher conversion, it was decided to increase the reaction temperature to 363 K. It was found that the TBHP conversion carried out at 363 K is nearly identical to those obtained for the epoxidation carried out at 353 K. Therefore, 353 K can be considered as an optimum temperature for epoxidation of 1-octene catalyzed by the PBI.Mo catalyst. 3.2. Effect of Water Content on the Epoxidation of Alkenes Catalyzed by PBI.Mo. Since the commercially supplied TBHP is wet containing ∼30 wt % water, it was thought worthwhile to examine the use of this oxidant without removing the water and transferring it into toluene. From Figure 2, it is clear that the rate of cyclohexene epoxidation in the presence of water is slower than the reaction conducted with dry TBHP. Although the rate of reaction is slower in the initial stage, the conversion of TBHP to the epoxide gradually increases to ∼99%. In our previous study of cyclohexene epoxidation using “wet” TBHP with homogeneous MoO2acac2 catalyst, it was found that the amount of cyclohexene oxide formed reached a maximum and then decayed away.15 The initially formed epoxide was shown to undergo facile hydrolysis to the diol, possibly catalyzed by MoO2acac2 acting as a Lewis acid. In the case of the epoxidation of cyclohexene with wet TBHP catalyzed by heterogeneous PBI.Mo, no side reaction nor
3296 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009
Figure 2. Effect of water content on TBHP conversion to epoxide and reaction rate of PBI.Mo catalyzed epoxidation of alkenes (at 343 K, 5/1 molar ratio of alkene to TBHP, catalyst content 0.3 mol % Mo, and 400 rpm agitation speed, conversion of TBHP is related to 1,2-limonene epoxide).
formation of diol was found. In contrast to epoxidation of other alkenes the only influence of water on the PBI.Mo catalyzed epoxidation of cyclohexene is to slow down the reaction rate to some extent. However, very interesting results were obtained for limonene epoxidation. Figure 2 shows the TBHP conversion to 1,2limonene oxide achieved under the standard experimental condition but using wet TBHP instead of the anhydrous material. The corresponding results obtained using PBI.Mo and dry TBHP are also shown in Figure 1b. From these data, it can be seen that the initial reaction rate is slightly faster using wet TBHP but the amount of 1,2-limonene oxide formed reaches a maximum of 50% TBHP conversion and then slowly decreases to about 40%. Even though there is no formation of limonene epoxide after 100 min, the TBHP conversion still increases reaching 80% at the end of the experiment and tert-butanol, the usual byproduct, is formed. Another interesting feature which is observed during the wet epoxidation of limonene was the diastereoselectivity. In reactions when dry TBHP is employed, a 50:50 mixture of cis and trans diastereoisomers of 1,2-limonene oxide is obtained, while when using wet TBHP, a 50:50 mixture is obtained only for the first 60 min. After this time, the diastereoismeric ratio changes to give a 25:75 mixture (cis:trans) at the end of the experiment. This is almost certainly because the epoxide(s) formed undergoes hydrolysis to diols, and the reaction of the cis epoxide is faster than the trans epoxide on steric grounds. Epoxide(s) continues to be formed and hydrolyzed after ∼60 min, and the total amount present decreases slightly, but the composition drifts in favor of the less reactive trans diastereomer. This difference in reactivity of the two isomers has been rarely observed; however, this feature has been reported before by Salles et al.40 They found that the ring opening of limonene oxide with water catalyzed by β-ketophosphonate Mo(VI) complexes occurred mostly with the cis isomer while the trans isomer remained largely unconverted.40 Since the cis isomer opens selectively, it can provide a method for separating the trans diastereomer of the 1,2-limonene epoxide. Bearing this in mind, some further control reactions are planned to shed more light on these results since in contrast to the work of Salles et al.40 we have used a heterogeneous Mo(VI) catalyst that could be a technological advantage in the separation of cis/trans-1,2limonene epoxide.41 Two less reactive alkenes, e.g. R-pinene and 1-octene, display very poor if not negligible reactivity with TBHP containing
water (Figure 2). The total conversion of TBHP achieved at the end of the epoxidation of R-pinene using wet TBHP is 20%. However, 99% of the consumed TBHP is decomposed to tertbutanol. At the end of the experiment, hardly any detectable conversion of TBHP to R-pinene oxide is realized (99% TBHP conversion is achieved after 120 min while for catalyst levels of 0.3 and 0.6 mol % Mo, similar TBHP conversions are obtained after 40 min (Figure 3a). It is evident that increase in the catalyst level above 0.3 mol % Mo has little effect on the TBHP conversion, and so, 0.3 mol % Mo of PBI.Mo can be considered to be optimal for cyclohexene epoxidation at 353 K with a 5/1 molar ratio of cyclohexene to TBHP. For limonene epoxidation with 0.15 mol % Mo, 90% TBHP conversion is achieved after 240 min, while with 0.3 mol% Mo, 90% conversion is achieved after 120 min. However, the reaction rate is particularly high when 0.6 mol % Mo is used. In this case, 90% TBHP conversion to 1,2-limonene oxide is obtained after 60 min, and the reaction is completed after 100 min (Figure 3b). The rate dependence on catalyst level is therefore very clear and is the most noticeable among the studied alkenes. The results for R-pinene epoxidation (Figure 3c) are similar to those found with cyclohexene i.e., the TBHP conversion and reaction rate obtained for the two largest catalyst contents (0.3 and 0.6 mol % Mo) are nearly identical. Slightly higher reaction rate, especially at the beginning of the reaction, for lower Mo loading can be explained by an exothermic effect which at that stage of the reaction was very difficult to control. The final TBHP conversion at the end of the reaction time is 90% in each case. As for cyclohexene, it can be concluded that increasing the level of catalyst above 0.3 mol % Mo neither improves TBHP conversion nor the reaction rate. For epoxidation of the least reactive alkene, i.e. 1-octene, change of the PBI.Mo content does not significantly influence the TBHP conversion and reaction rate (Figure 3d). Unlike the situation found with the other alkenes the difference in TBHP conversion (∼10%) for 0.15 and 0.6 mol % Mo remains almost the same during the course of reaction. 3.4. Effect of the Alkene/TBHP Feed Mole Ratio on Epoxidation of Alkenes Catalyzed by PBI.Mo. Typically in medium- to large-scale alkene epoxidation processes, the reactions are run with a substantial excess of alkene in order to achieve high consumption of the oxidant, to avoid overoxidation and hence to deliver high selectivity toward the epoxide. In addition, this ensures that no explosion limit is reached. When the cyclohexene/TBHP mole ratio is increased to 10/ 1, more than 99% conversion of TBHP is achieved after 20 min of reaction, while for a 5/1 and 2.5/1 mol ratio it took about 50 and 120 min, respectively (Figure 4a). As already mentioned, excess of alkene can improve the selectivity of the reaction; however for limonene epoxidation, regardless of the feed mole ratio, the selectivity toward 1,2-
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Figure 3. Effect of Mo/TBHP ratio on TBHP conversion and reaction rate of PBI.Mo catalyzed epoxidation of alkenes: (a) cyclohexene, (b) limonene (conversion of TBHP is related to 1,2-limonene epoxide), (c) R-pinene, (d) 1-octene (at 343 K, 5/1 molar ratio of alkene to TBHP, 400 rpm agitation speed).
limonene oxide did not change in all three experiments and remained at the same value of 94%. It can be seen from Figure 4b that variation of the limonene/TBHP mole ratio does not affect the TBHP conversion to 1,2-limonene oxide significantly. Unexpectedly, the slightly higher reaction rate obtained using a mole ratio of limonene/TBHP of 2.5/1 might be explained by a temperature increase (exothermic effect) that was observed when fresh catalyst was used. Similar studies in the epoxidation of R-pinene show that a mole ratio of R-pinene/TBHP of 5/1 can by considered as an optimal one. The TBHP conversion as well as the reaction rate obtained when 5/1 and 10/1 ratios are used are very similar. However, the initial reaction rate when using a 5/1 ratio is slightly higher (Figure 4c). The feed mole ratio of 2.5/1 (TBHP: R-pinene) gives a distinctly lower conversion and reaction rate compared to those obtained using higher ratios. In the case of 1-octene epoxidation under different 1-octene/ TBHP molar ratios, we have obtained unexpected results. After using 2.5/1 and 5/1 alkene/TBHP mole ratios, we observed that the TBHP conversion and the reaction rate were very similar for both cases (Figure 4d). These results were not entirely surprising since we had already noticed similar behavior in the limonene epoxidations. However, when a 1-octene/TBHP mole ratio of 10/1 is employed, surprisingly the TBHP conversion and reaction rate both decreased markedly This experiment was repeated several times, but similar results were obtained for all cases. Furthermore when using a 1-octene/TBHP mole ratio of 15/1, the final conversion of TBHP drops to only 21% (Figure 4d).
In explaining why an increase in the 1-octene/TBHP mole ratio leads to a decrease in the TBHP conversion and reaction rate, it was interesting to find out what happened when a ratio below that of the normal range was employed. In fact the results obtained for a reaction using a 1.25/1 mol ratio are very similar to those obtained using mole ratios of 5/1 and 2.5/1. Clearly the effect of the alkene/TBHP feed ratio is alkene dependent, and in the case of a rather unreactive alkene, such as 1-octene, it seems that increasing this ratio beyond 5/1 decreases the TBHP concentration to such an extent as to cause a reduction in the rate of conversion of TBHP to epoxide. Similar unpublished results showing a decrease of TBHP conversion with an increase in alkene/TBHP feed mole ratio have recently been obtained in our laboratory for PBI.Mo catalyzed epoxidation of 1-dodecene, another rather unreactive alkene. It seems therefore that this effect is characteristic of such alkenes. 3.5. Catalyst Reusability Studies of PBI.Mo in Epoxidation of Alkenes. In order to check the suitability of the catalyst for continuous use in the reactive distillation (RD) studies, it is essential to study the catalyst reusability in batch reactions using conditions likely to be used in the RD rig. In the batch epoxidations of all the alkenes reported so far, some attrition of catalyst particles was noticed when they had been reused more that three times under the stirring conditions used in the batch reactor. However it is anticipated that this problem will be absent with the catalyst in a fixed bed in the RDC rig. The data for cyclohexene epoxidation shown in Figure 5a indicate that the PBI.Mo catalyst remains active over many cycles. The high reaction rate in run 1 might be explained by
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Figure 4. Effect of feed mole ratio (alkene/TBHP) on TBHP conversion and reaction rate of PBI.Mo catalyzed epoxidation of alkenes: (a) cyclohexene, (b) limonene (conversion of TBHP is related to 1,2-limonene epoxide), (c) R-pinene, (d) 1-octene (at 343 K, catalyst content 0.3 mol % Mo, 400 rpm agitation speed).
the significant exothermic effect which is observed when fresh catalyst is used. Although the rate of reaction is slower when the catalyst is reused more than two times, nearly quantitative conversion of TBHP was obtained at the end of the experiment for all the runs. In the case of limonene epoxidation more than 90% yield of 1,2-limonene oxide is achieved at the end of experiments for all runs (Figure 5b). The rate of epoxide formation seems to decrease for the first three runs; however, it becomes stabilized after run 3 with the yield and reaction rate for run 3 and run 4 being nearly identical. For all the runs, the selectivity toward 1,2-limonene oxide remains constant during the course of reaction and is found to be 94% at the end of each experiment. The results from similar studies of R-pinene epoxidations are different from that obtained for cyclohexene and limonene. As is shown in Figure 5c, the TBHP conversion and reaction rate decrease gradually but the difference between consecutive runs, especially after run 3, is more significant than was recorded for cyclohexene and limonene. The conversion curves for runs 4 and 5 look very similar, and the final conversion for both experiments is low (∼30%). Therefore, it can be concluded that the activity of the PBI.Mo catalyst in the epoxidation of R-pinene remains stable after run 4, but this activity is decreased significantly compared to that in the first three runs. In the case of 1-octene (Figure 5d), analogous experiments show a significant decrease in the reaction rate and final TBHP conversion after run 1. The final TBHP conversion in run 2 drops by ∼40% compared to run 1, but in subsequent reactions, the activity decreases very slowly and finally shows stable behavior for runs 4-6.
In our previous work with other alkenes, we found that Mo loss in the first run using PBI.Mo could be in the range of 2-5%.42 This might be due to the presence of some weakly bound or physically trapped Mo complex. The high reactivity with fresh catalyst might therefore be attributed to leaching of this weakly bound Mo thereby catalysis of epoxidation in a homogeneous manner. In the present study when the PBI.Mo catalyst is used for a run 2 with 1-octene, the amount of Mo leaching to the system may well be less. Hence, the TBHP conversion achieved might be lower because the reaction is catalyzed mostly by heterogeneous Mo species. For runs 2-6, therefore, the amount of soluble Mo species present in the reaction mixture (if any at all) does not significantly influence the TBHP conversion. It seems clear therefore that catalysis in the early reactions arises from the heterogeneous Mo complexes immobilized on each support and active Mo species lost from the support. However, once the catalyst has been recycled the contribution to catalysis from the Mo species lost from the support become negligible. Overall, however, the activity of the PBI.Mo catalyst in 1-octene epoxidation is significantly lower than that observed with the other more electron rich alkenes. 3.6. Supernatant Studies for 1-Octene Epoxidation Catalyzed by PBI.Mo. In our earlier work,19 the limit of detection of our Mo analytical procedure was 0.5 ppm which would correspond to 0.2 wt % loss of Mo from the polymer catalysts. Recycling the latter a few times led to supernatant solutions in which no Mo was detectable, i.e. the Mo lost from the supports was
